1. Field of the Invention
The present invention relates to an epitaxially grown compound semiconductor film, and more particularly to an epitaxially grown compound semiconductor film of reduced crystal defect density. The invention also relates to a compound semiconductor multi-layer structure including the epitaxially grown compound semiconductor film. The invention further relates to a light-receiving device including the compound semiconductor multi-layer structure.
2. Description of the Prior Art
When an epitaxially grown semiconductor film containing crystal defects is employed in a device, reliability of the device is adversely affected. Therefore, demand has arisen for an epitaxially grown compound semiconductor film of reduced crystal defect density.
In general, compound semiconductor single-crystal substrates contain crystal defects. In order to grow a compound semiconductor multi-layer film on such a defect-containing compound semiconductor single-crystal substrate, when the substrate is brought into a crystal growth apparatus, the substrate is subjected to surface treatment. However, difficulty is encountered in completely removing defects present in the surface of the substrate, an oxidized layer formed on the surface, or impurities deposited or adsorbed onto the surface.
Therefore, in a conventional process, a buffer layer having a thickness of about 1 xcexcm is formed on a compound semiconductor single-crystal substrate, and subsequently a compound semiconductor multi-layer film is formed on the buffer layer. The buffer layer can lower the adverse effect of, for example, defects in the surface of the substrate on the multi-layer film. Also, the buffer layer reduces propagation of lattice defects contained in the substrate.
When a film of a compound semiconductor whose lattice constant varies with its compositional proportions is formed, regulation of the compositional proportions is a critical factor for determining the quality of the film. Particularly when a thick compound semiconductor film is formed, characteristics of the film are greatly affected by the difference in lattice constant between the film and a substrate, for the following reason. Since the thickness of the film is considerably smaller than that of the substrate, the difference in lattice constant between the film and the substrate causes generation of strain in the film. In order to prevent generation of such strain, in some cases, a buffer layer is formed on the substrate from the same material as the substrate, and then a film of intended structure is formed on the buffer layer.
Propagation of defects in a film grown on a substrate, the defects being generated by propagation of defects in the substrate, is prevented at, for example, the interface between layers formed of different semiconductor materials. This is because, when a film is grown on a substrate or an underlying layer such that the crystal structure of the substrate or underlying layer is reflected on the film, the thus-grown film exhibits microstructural discontinuity at the interface between layers formed of different semiconductor materials. In order to utilize such a phenomenon, in some cases, a buffer layer of superlattice structure is provided. The term xe2x80x9csuperlattice structurexe2x80x9d refers to a structure in which layers formed of at least two different materials are alternately laminated, and in many cases, the thicknesses of the layers are determined to be 20 nm or less.
As described above, various techniques which have heretofore been proposed for preventing generation of defects in a semiconductor film are not fully satisfactory, and depending on the device to which the film is incorporated, the structure of the device prevents employment of such techniques.
As described above, propagation of defects is prevented at the interface between layers formed of different semiconductor materials. However, in some cases, because of microstructural discontinuity at the interface, defects are newly generated at the interface.
Devices incorporating an epitaxially grown semiconductor film include a light-receiving device employed for optical communication. For example, a light-receiving device sensitive to light having a wavelength of 1.3 xcexcm or 1.55 xcexcm, which is generally employed for optical communication, has a structure including an InP substrate, and an InGaAs light-absorbing layer formed on the substrate. When reverse bias is applied to the light-receiving device in the absence of light, dark current flows through the device. The value of dark current varies in accordance with the structure of the light-receiving device, strain in the InGaAs layer, and the defect density of the layer. In order to reduce strain or defects in the InGaAs layer, when, for example, a conventional superlattice-structure layer is employed, a radical modification must be effected to the structure of the device. However, modification of the device structure causes variation in characteristics of the device, and therefore such modification is desirably avoided.
In view of the foregoing, an object of the present invention is to reduce the density of crystal defects in a compound semiconductor film, which defects are generated during the course of epitaxial growth of the film or in a process for producing a device, to thereby improve characteristics of the device including the epitaxially grown semiconductor film.
Another object of the present invention is to provide an epitaxially grown compound semiconductor film of reduced defect density.
Still another object of the present invention is to provide a compound semiconductor multi-layer structure comprising the epitaxially grown compound semiconductor film.
Yet another object of the present invention is to provide a light-receiving device comprising the compound semiconductor multi-layer structure.
According to a first aspect of the present invention, there is provided an epitaxially grown compound semiconductor film, wherein the compositional proportions of a compound semiconductor constituting the film cyclically vary in a thickness direction. Preferably, the profile of the cyclic variation of the compositional proportions in a thickness direction assumes the form of a sinusoid.
According to a second aspect of the present invention, there is provided a compound semiconductor multi-layer structure comprising such an epitaxially grown film. One embodiment of the compound semiconductor multi-layer structure is a multi-layer structure comprising a compound semiconductor substrate; a compound semiconductor buffer layer formed on the compound semiconductor substrate, the buffer layer being of the same conduction type as the compound semiconductor substrate; an undoped compound semiconductor light-absorbing layer formed on the compound semiconductor buffer layer; and a compound semiconductor cap layer formed on the compound semiconductor light-absorbing layer, the cap layer being of the same conduction type as the compound semiconductor substrate, wherein the compositional proportions of a compound semiconductor constituting the compound semiconductor light-absorbing layer cyclically vary in a thickness direction with respect to predetermined compositional proportions that establish lattice matching between the compound semiconductor and a compound semiconductor constituting the compound semiconductor substrate.
Another embodiment of the compound semiconductor multi-layer structure is a multi-layer structure comprising an InP substrate; an InP buffer layer formed on the InP substrate, the buffer layer being of the same conduction type as the InP substrate; an undoped InGaAs light-absorbing layer formed on the InP buffer layer; and an InP cap layer formed on the InGaAs light-absorbing layer, the cap layer being of the same conduction type as the InP substrate, wherein the compositional ratio between In and Ga which constitute the InGaAs light-absorbing layer cyclically varies in a thickness direction with respect to a predetermined compositional ratio that establishes lattice matching between InGaAs and InP constituting the substrate.
According to a third aspect of the present invention, there is provided a light-receiving device comprising such a compound semiconductor multi-layer structure. One embodiment of the light-receiving device is a light-receiving device comprising an n-type InP substrate; an n-type InP buffer layer formed on the InP substrate; an undoped InGaAs light-absorbing layer formed on the InP buffer layer; an n-type InP cap layer formed on the InGaAs light-absorbing layer; a Zn diffusion region formed in at least the InP cap layer; a p-type electrode formed on the upper surface of the Zn diffusion region; and an n-type electrode formed on the bottom surface of the InP substrate, wherein the compositional ratio between In and Ga which constitute the InGaAs light-absorbing layer cyclically varies in a thickness direction with respect to a predetermined compositional ratio that establishes lattice matching between InGaAs and InP constituting the substrate.